Method of making austenite type stainless steel

ABSTRACT

A method for rotary casting of austenitic stainless steels. When rotarily cast, steel ingots are likely to crack on the surface. In order to prevent cracking, it is desirable to adjust the Nickel balance to within a certain value. In stopping the rotation, ghost rings can be prevented by a gradual deceleration of the speed of rotation.

United States Patent 11 1 Uemura et a1.

Assignee: Sumitomo Metal Industries, Limited,

Osaka City, Japan Filed: Feb. 24, 1972 Appl. No.: 229,103

Related U.S. Application Data Continuation-impart of Ser. No. 840,610, July 8, 1969, abandoned.

U.S. Cl. 75/130.S, 164/114 Int. Cl. C22c 33/00, B22d 13/00 Field of Search 75/130.5; 148/38;

1451 Aug. 7, 1973 [56] References Cited UNITED STATES PATENTS 1,131,697 3/1915 Hess ..164/144 X 1,807,536 5/1931 Keup t 164/115 2,597,173 5/1952 Patterson 75/130.5 2,784,083 3/1957 Linnert et al. 148/38 X 3,352,666 ll/1967 Foster et a1 75/125 Primary ExaminerL. Dewayne Rutledge Att0rneyWats0n et al.

[57] ABSTRACT A method for rotary casting of austenitic stainless steels. When rotarily cast, steel ingots are likely to crack on the surface. In order to prevent cracking, it is desirable to adjust the Nickel balance to within a certain value. In stopping the rotation, ghost rings can be prevented by a gradual deceleration of the speed of rotation.

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METHOD OF MAKING AUSTENITE TYPE STAINLESS STEEL CROSS REFERENCE TO RELATED APPLICATION:

This is a continuation-in-part application of our copending application, Ser. No. 840,610, filed July 8, 1969, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an improved method for casting austenitic stainless steel ingots.

2. Description of the Prior Art Generally, when molten metal is cast in a rotating mold, the centrifugal forces will urge the molten metal to penetrate into cracks on the inside surface of the mold where the metal will immediately solidify to form imperfections. Further, during the solidification of the molten steel, shrinking forces caused by contraction act on the steel ingot to generate surface cracks. The size and number of cracks on the inside surface of the mold increase with an increase in the number of times the mold is used. This results in an increase in the frequency of the generation of cracks and flaws on the surface of the steel ingot.

However, the frequency of generation of such cracks and flaws on the surface of the steel ingot may be influenced more by the composition of the stainless steel than by the state of the inside surface of the casting mold. Thus, the greater the Nickel balance [that is, the ratio of N equivalent (N equ) to N equivalent (N equ) as defined hereinafter] in the stainless steel, the wider the solidifying temperature range of the steel. Accordingly, when the molten steel begins to solidify, there is a greater tendency for liquid to remain between crystal grains which have already formed whereby the grains may separate from each other to develop into cracks and flaws on the surface of the steel ingot.

Further, the thermal conductivity of stainless steel varies greatly with the composition of the steel. An increase in the Nickel balance causes thermal conductivity to decrease. Therefore, in an austenitic stainless steel in which the Nickel balance is high, the temperature gradient within the steel ingot will be steep causing a high degree of internal strain within the ingot as it solidifies. This increases the likelihood that the ingot will break.

For the above reasons, the casting of austenitic stainless steel in rotary molds has often resulted in excessive generation of surface cracks and flaws in the lower parts of steel ingots and has caused various other problems, for example, rolled billets which have too many defects and must be reworked whereby product yields are reduced.

When the cross section of an austenitic stainless steel ingot, which has been cast in a rotating mold, is etched with a warm aqueous solution of hydrochloric acid, a ring-shaped corroded part, having a width of a few millimeters, will result. Such ring will not vanish even when the ingot is hot-rolled into a round billet, or is further extruded or cold-reduced. In some cases, these rings are said to evidence the fact of rotary casting. However, the exact mechanism of the production of ghost rings has not yet been fully identified.

SUMMARY OF THE INVENTION An object of the present invention is the provision of a rotary casting method wherein ghost rings are prevented by a gradual deceleration of the speed of rotation during stopping operations.

Another object of the present invention is the provision of a method wherein the incidence and magnitude of cracks and flaws on the surface of an austenitic stainless steel ingot cast in a rotating mold, are reduced whereby the quality and yield of product is improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photograph illustrating the cross-sectional macro structure of a cast stainless steel ingot produced utilizing conventional rotary casting methods wherein the duration of rotation was about 10 min. after cast- FIG. 2 is a photograph illustrating the cross-sectional macro structure of a stainless steel billet which was rotated twice during solidifaction;

FIG. 3 is a diagram illustrating the relationship between the Nickel balance and the presence of ghost rings;

FIG. 4 is a diagram illustrating the temperature distribution adjacent the solidifying front; FIG. 5 is a photograph illustrating the cross-sectional macro structure of an austenitic stainless steel billet produced utilizing the method of the present invention; and

FIG. 6 is a diagram illustrating a deceleration curve useful for production of an austenitic stainless steel ingot in accordance with the method of the present invention.

DESCRIPTION OF THE PREFERRED v EMBODIMENT With reference to the accompanying drawings, FIG.

1 illustrates the cross-sectional macro structure of an austenitic stainless steel ingot which was cast by a conventional method and which was thereafter etched with a warm aqueous solution of hydrochloric acid. It is known that when the rotating time is very short (about 10 min. compared to a total solidifying time of about 2 hrs.), when the rotation is stopped some phenomenon, which occurs at the solidifying front within the steel ingot, contributes to the production of a ghost ring. Utilizing various identification tests, three theories have evolved to explain this phenomenon: (A) that non-metallic inclusions, which have been moving under the influence of centripetal force, are accumulated in a fixed location when the rotation stops and such locations are susceptible to corrosion; (B) that the direction of growth of columnar crystals during rotation is different than when the ingot is stopped and as a result strains are produced on the surface thereby rendering the surface susceptible to corrosion; and (C) that, due to some unknown cause, 8 ferrite is deposited at the solidifying front during the time the ingot is stopped and such deposit produces a ghost ring which remains in the final product.

However, as a result of making a detailed micro examination of the distribution of non-metallic inclusions in a rotary cast ingot, no abnormal distribution of inclusions has been found. Further, as is shown in FIG. 2, when the ingot was rotated twice, three ghost rings were generated. Thus it is hypothesized that a ghost Revolutions per Process Second During Casting "W 0.85 minutes after casting 1.0 l0 to 30 minutes after casting Stopped 30 to 45 minutes after casting 1.0 45 minutes after casting Stopped As shown in FIG. 2, reference numeral 1 designates the ring generated during the first stoppage of rotation; numeral 2 designates the ring generated during the restarting of rotation; and numeral 3 designates the ring generated during the second stoppage of rotation. The ghost rings are clearly apparent. Even in theory, since the moving speed of non-metallic inclusions would most likely be related to their size, large numbers of the inclusions would have to be of the same size or such distribution could not occur. Thus, theory (A) set forth above appears to be unsound.

As can be seen in FIG. 1, even where there is a ghost ring, there does not appear to be a difference in the directivity of columnar crystals. Even if it were to be assumed that the ghost ring is generated by a discontinued surface of a columnar crystal, it would not follow that such ghost ring would remain even after the structure is subjected to a subsequent hot rolling step. Thus the grounds upon which theory (B) is based are questionable.

Therefore, it is concluded, that in all probability, ghost rings are caused by the deposition of 8 ferrite as suggested in theory (C) set forth above.

Further, as a result of further identification tests, ghost rings have been identified as comprising a ferrite deposit in view of the fact that when a billet having a ghost ring is heat-treated at 1300" C. for 2 hrs. and is then water-cooled, the ghost ring will completely vanish. Also, if the apparent visual intensities of the ghost rings, as judged with the naked eye, are categorized in three classes of increasing intensity (A B C) and are correlated with the Nickel balance, visual intensity, as can be seen in FIG. 3, decreases with an increase in the Nickel balance. From this it can be concluded that ghost rings are caused by the segregation of 8 ferrite.

When rotation is stopped, the solidified portions of the ingot will stop immediately along with the casting mold. However, the still liquid portion in the central part of the steel ingot will continue to rotate. This will cause turbulent flow and agitation at the boundary between the liquid and solid portions. Therefore, it is presumed, as discussed below that the 8 ferrite deposition will occur near the solidifying front.

The temperature distribution within an ingot near the solidifying front is as shown in FIG. 4. As a result, as shown in FIG. 4 (C), shortly after revolution has been stopped, temperature drops A t, and A t, occur respectively in the parts (a) and (b). Thus, it is presumed that part (a) will be quenched and the 5 ferrite generated during the initial period of solidification will remain without being transformed to austenite. On the other hand, part (b) will likely be transformed from 6 ferrite to austenite and will appear as a ring-shaped zone which can be judged by macro etching. FIGS. 4 (A), (B) and (C) show the temperature distributions in the ingot near the solidifying front, respectively: (1) just before rotation is stopped; (2) just after rotation is stopped; and (3) after normal rotation is reestablished. In the diagrams, S represents the completely solid state; L represents the completely liquid state: ST represents the maximum complete solidification temperature; and LT represents the minimum complete liquification temperature. It should be appreciated that, during rotation, turbulence within the steel ingot is generally minimal, as in the case of stationary casting. Thus, during rotation, composition segregation during solidification is essentially only micro segregation between crystal grains. However, when the solidified surface is washed by a turbulent flow of still liquid steel at the time rotation is stopped, macro segregation will occur. Macro segregation is likely to produce 8 ferrite and the resultant ghost ring.

It has been discovered, during research into the causes and methods for preventing surface cracks in austenite type stainless steel ingots, that, during rotary casting processes where molten steel is rotated for more than 3 min. in accordance with the formula W K/ V7 (wherein K has a value in a range of 3.3 to 50; W is the speed of rotation in revolutions per second and r is the radius of the steel ingot in millimeters), surface cracks in steel ingots can be prevented if the steel composition is adjusted such that the presence of ferrite in the steel is minimized consistent with the production of an austenitic stainless steel. In this connection it must be appreciated that if less than 2 percent ferrite is present in the ingot, the solidifying temperature range will be narrower than preferred in austenitic stainless steel and the thermal conductivity will be greater than desirable. On the other hand, any ferrite present in the steel may be easily converted to the sigma phase when heated to a high temperature and therefore, it is necessary to minimize the amount of ferrite in the ingot to prevent surface cracks.

According to the present invention, surface cracks in steel ingots can be minimized by adjusting the composition such that its Ni equ is equal to or less than its Ni equ when Ni equ 1.35 (Cr% 15 Si% Mo% 0.5 Nb% 1.25 Ti%) 11.40

and

Ni equ Ni% 30C% 0.5 Mn% wherein Cr%, Si%, Mo%, Nb%, Ti%, Ni%, C%, and Mn% are respectively the percentages of Chromium, Silicon, Molybdenum, Nobium, Titanium, Nickel, Carbon and Manganese in the austenitic stainless steel. In these formulas, it should be appreciated that C and Mn are austenite formers while Si, Mo, Nb and. Ti are ferrite formers. In each case the percentages have been converted to the Ni equivalent through the use of an appropriate constant.

The present invention shall be explained below in connection with the examples set forth in Table I wherein Nos. I,2,3 and 4 represent conventional steels in which the Ni equ is less than the Ni equ and wherein Nos. 5,6,7 and 8 represent steels useful in connection with the present invention in which Ni,equ is greater than the Ni,equ. In each instance, molten steel was cast utilizing a rotational speed of 1.0 revolution per second for 10 min. Each resultant steel ingot was 600 millimeters in diameter X l,500 millimeters high and weighed approximately 3,000 kilograms.

ing casting has an entirely uniform structure as is clearly shown in FIG. 5 in which the cross section has been acid etched using the same procedure as de- TABLE I Percent Surface Si Mn Ni Cr M0 Ti Ni; equ N i; equ Cracks Conventional mothotl, No

0. 07 0. 58 1. 51 12. 10 17. 40 0. 12 0. 41 14. 11 14. 96 Very large 0. 00 0. 48 1. 51 12. 00 17. 00 0.12 0. 53 13. 58 14. 56 Do. 0. 08 0. 61 1. 48 12. 10 17. 40 0. 11 0. 40 14.15 15. 24 D0. 0. 08 0. 51 1. 60 20. 51 25. 00 0. 06 23. 47 23. 71 Large.

0. O6 0. 71 1. 55 11. 2A) 17.75 0. 0.50 14. 91 13. 78 None. 0. 06 0. 08 1. 60 11. 20 17. 90 0. 05 0. 46 14. 09 13. 80 D0. 0. 07 0. 63 1. 46 11. 40 17. 60 0. 05 0. 51 14. 56 14. 23 D0. 0. 07 0. 64 1. 45 20. 65 25. 05 0. 04 23. 76 23. 48 D0.

From Table 1, it can be seen that, when the Ni,equ is greater than the Ni equ, no surface crack occurs in the steel ingot.

In order to prevent ghost rings in steel ingots pro-- duced by rotary casting, it would be desirable (1) to increase the ratio of the N i equ (austenite formers) relative to the Ni equ (ferrite formers) in the steel ingot and (2) to subject the ingot to heat-treatment after casting. However, each of these methods increases the production cost and the first method mentioned above the reference letters A to E in FIG. 6 after being subjected to a standard rotation time of min. following casting. In case B, the ingot was rotated at a standard rotation speed of l revolution per second for 10 min. This was followed by a deceleration period during which the speed of rotation was continuously and gradually decreased. A constant deceleration rate of 0.14 to 0.17 X 10 revolutions per second was found to be most effective. Further, linear deceleration rate, at the solidifying front, of less than about 0.27 centimeters per second has been found to be preferred. This would occur, for example, at a deceleration rate of 0.17 X

10 revolutions per second if the diameter of the so-' lidifying front were 50.6 centimeters and at a deceleration rate of 0.14 X 10 revolutions per second if the diameter of the solidifying front were 61.5 centimeters. The preferred linear deceleration rate can be calculated from a given angular deceleration rate and diameter of solidifying front.

A stainless steel ingot obtained in accordance with the present invention by slowly decelerating the rotatscribed in connection with FIG. 1. The method of the present invention can be very easily applied by utilizing a simple programmed rotation controlling device in connection with a conventional rotary casting apparatus.

, Through the use of the present invention, the occurrence of surface cracks and flaws in the surface of rotary cast austenitic stainless steel ingots is practically eliminated whereby the quality and yield of steel products are increased and conditioning steps are minimized.

We claim:

1. In a method for rotarily casting ingots from austenitic stainless steels, the steps of:

preparing a molten batch of austenitic stainless steel and maintaining the Ni equ of the batch at a level equal to or less than the Ni equ thereof, said Ni equ being equal to 1.35(Cr% 1.5 Si% Mo% 0.5 Nb% +1.25 Ti%) 11.40 and said equ being equal to Ni% 30 C% 0.5 Mn%; and

rotarily casting an ingot from said molten batch by rotating the same for a period of time greater than three minutes at a predetermined speed of rotation detennined by the relationship W K/ r wherein W is the speed of rotation in revolutions per second, r is the radius of the ingot in millimeters and K is a factor having a value in the range of 3.3 to 50.

2. A method as set forth in claim 1 wherein is included the step of slowly and continuously decelerating the batch at a deceleration rate less than about 0.27 cm/sec at the solidifying front in the batch only after said batch has been rotated at said speed of rotation for said period of time. 

2. A method as set forth in claim 1 wherein is included the step of slowly and continuously decelerating the batch at a deceleration rate less than about 0.27 cm/sec2 at the solidifying front in the batch only after said batch has been rotated at said speed of rotation for said period of time. 